This invention relates to manufacture of a bulky, soft and absorbent paper web and is particularly applicable to the manufacture of sanitary paper and similar products, such as tissue, toweling and similar papers wherein these characteristics are particularly desirable.
Conventional papermaking techniques are generally unsatisfactory for the manufacture of bulky, soft and absorbent paper webs since they utilize prior to drying one or more pressing operations on substantially the entire surface of the paper web to expel excess water, smooth the sheet and provide strength thereto. While overall pressing operations are probably the most efficient methods of dewatering and achieve requisite tensile strength in a paper sheet by bringing the individual fibers of the web into close physical proximity, this operating efficiency and increase in product strength are more than counter-balanced by destroying the desirable combination of softness, absorbency and bulk desirable in sanitary and similar products.
A number of approaches have been developed in an attempt to provide a soft, bulky and absorbent sheet which has desirable strength characteristics. One approach is set forth in U.S. Pat. No. 3,301,746 which discloses a process wherein a paper web is formed with essentially no pressing, is thermally pre-dried, and is then heavily compacted in a knuckle pattern against a dryer drum while the web is still wet enough to allow increase in bonding by compaction.
Pre-drying of the web shown in U.S. Pat. No. 3,301,746 is accomplished by means of a through-drying system in which hot gases are passed through the web prior to imprinting of the knuckle fabric at the dryer drum. Water removal using through-dryers of the type utilized in the U.S. Pat. No. 3,301,746 process is very energy intensive and such dryers are constructed and operated at considerable expense.
Other systems have been developed for the manufacture of an absorbent, soft, bulky web. One such arrangement is disclosed in U.S. Pat. No. 3,812,000 directed to a process wherein an elastomeric bonding material is mixed with lignocellulosic fibers and the web formed thereby is dried without mechanical compression until it is at least 80% dry. The various techniques taught for drying the web are radiant heat lamps, tunnel dryers, or transpiration dryers (through-dryers) wherein air, preferably heated, is used to pre-dry the web. Again, use of such pre-drying techniques results in a considerable expenditure of energy with consequent high expense.
Yet another technique is taught in U.S. Pat. No. 3,821,068. According to the technique taught in this patent a creped web is formed by deposition from an aqueous slurry of principally lignocellulosic fibers and dried to at least 80% fiber consistency or dryness without being subjected to mechanical compression of the web to substantially reduce formation of papermaking bonds which would form upon compression of the web while wet. A creping adhesive is applied to one surface of the web and the web is adhered to a creping surface with the web being dried on the creping surface to about 95% dryness level before removal with a creping blade. Pre-drying is again effected thermally through the use of radiant heat lamps, tunnel dryers, or preferably transpiration dryers. As stated above, such pre-drying arrangements are energy intensive and expensive.
All of the above-described methods operate on a common assumption, that is, that pre-drying or dewatering without mechanical compression is necessary to produce a product having adequate strength and yet having the desirable characteristics associated with sanitary papers and the like of softness, bulk and absorbency. Since mechanical compression is avoided in the pre-drying stage various expensive pre-drying substitutes must be adopted to preliminarily dewater the sheet without compacting same. It will be appreciated that such through-drying techniques are wasteful in that they use up inordinate amounts of diminishing natural resources such as natural gas.
It is therefore an object of the present invention to provide a process for manufacturing a soft, bulky and absorbent paper sheet having adequate strength and of a quality at least comparable to the products produced by the above-described prior art systems without the use of thermal pre-drying techniques. In view of the teachings of the prior art, it is very surprising to learn that a sheet of the type desired can be produced through the use of a mechanical dewatering or pre-drying step. It is perhaps even more surprising to learn that the process of this invention may be used to produce a web whose bulk is actually enhanced by passing it through a compression nip of a specific nature.